Permeability of the sheep placenta to unmetabolized polar non‐electrolytes.

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Abstract
1. The rate of appearance in uterine venous blood of radioactively labelled, polar, non‐electrolytes has been measured following their injection into the foetal circulation in the chronically catheterized sheep near term. Uterine blood flow was measured by an antipyrine technique. 2. Estimates of a placental permeability constant not corrected for area were 61‐2 +/‐ 5‐5 ml. min‐1 (mean +/‐ S.E. of mean) for [14C] urea, 1‐85 +/‐ 0‐16 for [14C]erythritol and 0‐21 +/‐ 0‐03 for [3H]mannitol. Results are also presented for [14C]ethylene glycol, [14C]L‐glucose, [14C]mannitol, [51Cr] EDTA and [3H] and [14C]sucrose. 3. In four sheep, permeability measurements for several solutes were made and the results were analysed in terms of restricted diffusion via cylindrical, water‐filled pores. Calculation of pore radius was made by a minimum variance method and values ranging from 0‐43 to 0‐45 nm were obtained. 4. Stability and absence of protein binding of probe molecules was investigated by gel permeation on Sephadex columns.